Dear friend
I have looked into your file, there are many things that need to be reconsidered here.
1. material assignment: everything in the design is assigned copper, even the radiation box
2. port assignment: port is not assigned properly, you have assigned the port as a circle to the monopole's end while there is no contact to the ground plane.
3. monople dimensions: calculate the dimension of your monopole once again.
Fix these issues and you will be able to simulate the structure. Monopole's are one of the basic forms of antenna which doesn't need much analysis and easiest to build if done the right way
Regards
Elchiquito